The present invention relates to analog-to-digital converters and particularly to open-loop analog-to-frequency type converters.
In many applications, it may be necessary or desirable to convert an analog signal to a digital signal. For example, data is frequently generated in the form of an analog signal. However, such analog signals are difficult to handle, process, and store for later use without introducing considerable error. In addition, analog signals which must be taken from a large number of sources will often accumulate at a rate which makes analysis and processing very difficult. It is desirable, therefore, to convert the analog signals to digital signals to permit the high speed processing made possible utilizing a digital computer. Such an analog-to-digital conversion is accomplished with a device which accepts an analog voltage signal at its input and produces a coded digital signal as its output. Because of the numerous and varied applications for analog-to-digital converters, many techniques have been devised to meet the requirements of the particular application. Thus, numerous analog-to-digital converters exist ranging from very slow and inexpensive devices to ultrafast, very expensive ones.
Despite their number, A/D converters can be generally placed in one of two major groups. The first major group utilizes the feedback technique and includes ramp and counter methods, successive approximation, subranging, non-linear conversion and the double and triple ramp techniques to name just a few. The second major group are the open-loop analog-to-digital techniques which include analog-to-pulsewidth, simultaneous conversion, cascade (or voltage doubling) conversion and analog-to-frequency conversion. The present invention is in this last category - namely an open-loop, analog-to-frequency type converter. A comprehensive summary of the various techniques of analog-to-digital conversion are included in A/D Conversion Series -- Part 1, entitled "Analog-to-Digital Conversion Techniques" by Ed Renschler and published as "Application Note An-471" by Motorola Semiconductor Products, Inc. In particular, the analog-to-frequency concept generally utilized in the present invention is described on page 3. In general, a voltage-to-frequency converter circuit described produces a frequency which is a precise, linear function of the input analog voltage. This voltage-to-frequency converter signal is "ANDED" with a "fixed time base signal" which is a digital pulse whose ON time is known and must be very precise. In operation, the leading edge of the "fixed timebase signal" gates an oscillator frequency to a counter. After the counter has had the oscillator frequency applied to it for the duration of the fixed time-base signal, the trailing edge will gate off the counter, and its contents will be a digital representation of the analog input voltage.
While this technique may be adequate in some applications, accuracy limitations due to non-linearities in the voltage-to-frequency converter over the entire analog input range and the precision required in the time-base signal pulsewidth prevents its utilization in many applications. The present invention overcomes these difficulties by providing a stable reference voltage for the voltage-to-frequency converter which may be implemented utilizing essentially one precision part to circumvent the non-linearity problem. Furthermore, the dependence upon a time-base signal having a precise pulsewidth is eliminated by utilizing an oscillator or digital clock to synchronize the VFO and the time-gate generator so that the conversion of the voltage-to-frequency converter frequency output to a digital number is determined by a simple logic ratio. The overall accuracy is consequently a sole function of the oscillator's (digital clock's) short-term stability. As a result, the time gate required by the above reference to produce a precise "fixed time base signal" is eliminated.